An electrophotographic photoreceptor is repeatedly used during an electrophotographic process, that is, in cycles of charging, exposure, developing, transfer, cleaning, neutralizing, and the like. Various stresses applied to the photoreceptor in the process degrade the photoreceptor. As examples of such degradations, there are chemical and electric degradations such as: chemical damages given to the photosensitive layer caused by strongly oxidizing ozone or NOx generated from a corona charging device, which is a charger usually used, and decomposition of the photosensitive layer composition caused by carrier (current) generated during image exposure period flowing within the photosensitive layer or light, e.g., neutralizing light and light from outside. There are also mechanical degradations such as occurrence of wears, scratches, and peeling-off of the film on the photosensitive layer surface, and the like, which are caused by rubbing of a cleaning blade or a magnetic brush, contacts with a developer or a paper or the like. Such damages accompanied with mechanical degradations are apt to appear on the image and directly impair quality of the image, and thus serve as one of the major reasons that limit the life of the photoreceptor.
A photosensitive layer in a typical photoreceptor, which does not include a functional layer (e.g. surface protect layer), is particularly vulnerable to these loads. A photosensitive layer usually includes a binder resin, which substantially determines the hardness of the layer, and a photoconductive substance. But a relatively high dope amount of the photoconductive substance has been an obstacle to achieving a sufficient mechanical strength of a photosensitive layer so far. As a binder resin for a photosensitive layer, polyester resins having excellent sensitivity and a wear resistance have recently been used (refer to PTL 1 to PTL 8).
Meanwhile, although there are various known producing methods of polyester resins, interfacial polymerization has widely been used to obtain high-molecular-weight, less-colored, and highly pure polyester resins. Unfortunately, in a case where a polyester resin is produced from, for example, a readily oxidizable monomer such as hydroquinone using an interfacial polymerization method, the monomer is promptly oxidized in an alkaline aqueous solution and becomes an oxidized form of the monomer such as quinone. Such an oxidized form shows poor reactivity with a dicarboxylic acid chloride, therefore, it is difficult to incorporate an intended amount thereof into the resin, and further, the oxidized form remaining in the resin causes coloration of the resin. A method for preventing such oxidation of monomers during polymerization has been studied, and a polymerization method in which an excessive amount of an antioxidant or the like is added is known (refer to PTL 9).